Email filtering involves the processing of email for forwarding, redirecting or deletion according to predetermined criteria. Most often email filtering refers to the automatic processing of incoming email but can also involve human intervention as well. Email filtering software accesses email as inputs and as an output can either cause an email message to pass through the filtering process unchanged for delivery to a user's email message mailbox, redirect the email message for delivery elsewhere, or even throw the email message away.
Spammers send unsolicited bulk email or unsolicited commercial email that is referred to as “spam”. Spam can refer to the unsolicited bulk or commercial email itself or to its content. Spammers attempt to devise email messages that contain spam that can penetrate email filters and be delivered to targeted email users. Spammers use various techniques in order to fashion email messages that can penetrate an email filter. One approach taken by spammers involves running test messages through spam filters in order to determine the words and other email attributes that the spam filters consider to be legitimate. By adding sufficient numbers of words and attributes that are considered to be legitimate to an email message that contains spam, an email filter can be led to classify the email message as legitimate and to allow it to pass through to the email message mailbox of targeted users.
Some conventional spam filtering systems that use email message sender reputation information as a criteria for determining how an email filter handles an email message, provide email message senders with a reputation “score” that is used in conjunction with a content based “score” to form a combined score that is used to classify email messages from that email message sender. Accordingly, in such conventional spam filtering systems the reputation score of the email message sender only constitutes a part of the basis for a decision of how to filter an email message irregardless of the nature or magnitude of the reputation score accorded the email message sender: (1) good, (2) poor, or (3) neutral.
It should be appreciated that the aforementioned use of reputation information related to email senders can contribute to unsatisfactory filtering results as it relates to certain types of email senders. Another shortcoming of conventional email filtering systems is that all email is subjected to the same content filter, the same network policies (such as hourly throttling limits, connection limits, etc.), and the same delivery options: (1) inbox, (2) junk, or (3) delete in spite of reputation.
The character of some email reputation information is problematic for conventional email filtering systems such as email sender reputation information that is inconclusive. For example, reputation information for email senders who are very low volume senders can be weak and may not provide an accurate estimation of the likelihood that the email that the email sender is sending is undesirable. Moreover, reputation information for mixed email message senders who forward email messages, send gray mail (newsletters, legitimate advertisements) or forward aggregate data (e.g., newsgroups) is often inconclusive. Likewise, reputation information for infrequent email senders who send high volumes of email can be inconclusive.
Conventional email filters such as those which are discussed above can be frustrated by spammers who overload email messages subject to receiving a poor sender reputation score with legitimate content (e.g., content determined to be satisfactory or desirable for forwarding to a user). In this manner the spammers raise the overall score of the email message above that which qualifies it as legitimate. In addition, the lack of selectivity of conventional filters, as it regards email sent from senders with inconclusive reputations, results in excessive false positive identifications of bad email. This is because conventional email filters do not have the capacity to identify good email sent from such senders. It should be appreciated that such excessive false positives can result in the elimination of substantial amounts of desirable email. Accordingly, conventional email filters ineffectively use reputation information which significantly degrades their overall performance.